There are known binoculars and telescopes which consist of objectives and inversion systems for image erection. The objectives can be constructed of refracting lenses can comprise mirrors and the inversion systems are commonly constructed as lenses or prismatic inversion systems.
Further, a large number of reflector telescopes of different construction are known to be used for astronomical purposes.
The basic different types are:
the Newtonian telescope with reversed image, PA1 the Cassegrainian telescope with reversed image, and PA1 the Gregorian telescope with upright image.
Binoculars and telescopes with objectives made of lenses for medium or high-degree magnification could be constructed considerably shorter after the invention of the teleobjective; besides, in the past due to the use of prismatic inversion systems the systems have been either somewhat shorter and therefore wider, or still relatively long and therefore based on direct vision.
In the case of telescopes with lens inversion systems very tall construction necessarily results.
Since teleobjectives, especially due to the requirements of correcting the chromatic aberrations and of reducing the height of the construction, are made of a plurality of lenses, this type of construction including the prismatic inversion systems or the lens reversion systems has a cumbersome total weight meaning that binoculars or telescopes for medium or high-degree magnification cannot be comfortably carried around for a long time when traveling.
In the case of reflector telescopes such as the Newton telescope or the Cassegrain telescope, lens reversal systems or prismatic reversed systems are needed for the reversion of the image.
In contrast to the already-mentioned telescope types, the Gregorian telescope provides directly an upright image. This is possible because a real reversed intermediary image produced by the main mirror is reproduced by the collector mirror used as a reversal system, as an upright, real intermediary image.
Essential advantages of the Gregory telescope consist therefore of the upright image, the achromatic quality of the mirror and the relatively reduced weight. As disadvantages can be mentioned the relatively considerable over-all length and the short focal distance of the collector mirror in dependence upon the typical image ratio of .beta.=-4, which is unfavorable for the image quality. This because the sum of the focal distances of the main mirror must and the object distance of the reversing mirror be approximately equal to the image distance of the reversing mirror. This strong secondary magnification of the first intermediary image leads to an increase in image distortion of the main mirror and, on the other hand, even when the main mirror is parabolically shaped and the reversing mirror is elliptically shaped, additional strong extra-axial image distortions occur, due to the considerable ellipticity of the reversing mirror determined by the image proportion .beta.=-4. This, in addition to the relatively large over-all length, is regrettable for the Gregorian telescope, since otherwise it allows for a very light telescope construction because of the reduced number of components. A modification of the mirror focal distance is not possible in the Gregorian telescope, because the sum of the focal distance of the main mirror and the object distance of the reversing mirror must be approximately equal to the image distance of the reversing mirror.
The disadvantages of the known reflector telescopes are, therefore, that they either must have additional lens of prismatic reversing systems for image erection for ground observation (terrestrial observation), or have relative large over-all construction lengths or type-conditioned image aberrations.